Digital Watermarking

The enormous popularity of the World Wide Web in the early 1990's
demonstrated the commercial potential of offering multimedia resources through
the digital networks. Since commercial interests seek to use the digital
networks to offer digital media for profit, they have a strong interest in
protecting their ownership rights. Digital watermarking has been proposed as
one way to accomplish this.

A digital watermark is a digital signal or pattern inserted into a digital
image. Since this signal or pattern is present in each unaltered copy of the
original image, the digital watermark may also serve as a digital signature for
the copies. A given watermark may be unique to each copy (e.g., to identify the
intended recipient), or be common to multiple copies (e.g., to identify the
document source). In either case, the watermarking of the document involves the
transformation of the original into another form. This distinguishes digital
watermarking from digital fingerprinting where the original file remains intact,
but another file is created that "describes" the original file's
content. As a simple example, the checksum field for a disk sector would be a
fingerprint of the preceding block of data. Similarly, hash algorithms produce
fingerprint files.

Digital watermarking is also to be contrasted with public-key encryption,
which also transform original files into another form. It is a common
practice nowadays to encrypt digital

documents so that they become un-viewable without the decryption

key. Unlike encryption, however, digital watermarking leaves the original
image or (or file) basically intact and recognizable. In addition, digital
watermarks, as signatures, may not be validated without special software.
Further, decrypted documents are free of any residual effects of encryption,
whereas digital watermarks are designed to be persistent in viewing, printing,
or subsequent re-transmission or dissemination.

THE PURPOSE OF DIGITAL WATERMARKS

Two types of digital watermarks may be distinguished, depending upon whether
the watermark appears visible or invisible to the casual viewer. Visible
watermarks are used in much the same way as their bond paper ancestors, where
the opacity of paper is altered by physically stamping it with an identifying
pattern.

This is done to mark the paper manufacturer or paper type. One might view
digitally watermarked documents and images as digitally "stamped".

The visible watermarks which appear in Figures 1 and 2 illustrate the
technique. The watermark in Figure 1 appears is quite obtrusive because of the
high contrast between the background and foreground drawing. There is no place
for the watermark to "hide" as it were. The colored image in Figure 2
renders the visible watermark less obvious.

Figure 1.Digital Copy of fifteenth century drawing with
digital watermark superimposed.
Figure 2.Digitized copy of artwork from a sixteenth
century Aztec manuscript.

Invisible watermarks, on the other hand, are potentially useful as a means
of identifying the source, author, creator, owner, distributor or authorized
consumer of a document or image. For this purpose, the objective is to
permanently and unalterably mark the image so that the credit or assignment is
beyond dispute. In the event of illicit usage, the watermark would facilitate
the claim of ownership, the receipt of copyright revenues, or the success of
prosecution.

Watermarking has also been proposed to trace images in the event of their illicit redistribution.
Whereas past infringement with copyrighted documents was often limited by the unfeasibility of
large-scale photocopying and distribution, modern digital networks make large-scale
dissemination simple and inexpensive. Digital watermarking makes it possible to uniquely mark each
image for every buyer. If that buyer then makes an illicit copy,
the illicit duplication may be convincingly demonstrated.

VISIBLE VS. INVISIBLE WATERMARKS

Visible and invisible watermarks both serve to deter theft but they do so in
very different ways. Visible watermarks are especially useful for conveying an
immediate claim of ownership. The main advantage of visible watermarks, in
principle at least, is that they virtually eliminate the commercial value of the
document to a would-be thief without lessening the document's utility for
legitimate, authorized purposes. A familiar example of a visible watermark is
in the video domain where CNN and other television networks place their
translucent logo at the bottom right of the screen image.

Invisible watermarks, on the other hand, are more of an aid in catching the
thief than discouraging the theft in the first place.

TABLE 1 CAPTION: Visible and invisible watermarks act as a
deterrence to theft in different ways. Visible watermarks diminish the
commercial value of the document or image. Invisible watermarks increase the
likelihood of successful prosecution. Invisible watermarks may also act as a
deterrent if perpetrator is aware of their possible use.

REQUIREMENTS OF WATERMARKS

To be effective in the protection of the ownership of intellectual property,
the invisibly watermarked document should satisfy several criteria:

the watermark must be difficult or impossible to remove, at least
without visibly degrading the original image,

the watermark must survive image modifications that are common to
typical image-processing applications (e.g., scaling, color requantization,
dithering, cropping, and image compression),

an invisible watermark should be imperceptible so as not to affect the
experience of viewing the image, and

for some invisible watermarking applications, watermarks should be
readily detectable by the proper authorities, even if imperceptible to the
average observer. Such decodability without requiring the original,
un-watermarked image would be necessary for efficient recovery of property and
subsequent prosecution.

One can understand the challenge of researchers in this field
since the above requirements compete, each with the others. The
litmus test of a watermarking method would be that it is accepted
and used on a large, commercial scale, and that it stands up in a
court of law. None of the digital techniques have yet to meet
these tests.

TECHNIQUES FOR WATERMARKING

Watermarking techniques tend to divide into two categories, text and image,
according to the type of document to be watermarked. Techniques for images: Several
different methods enable watermarking in the spatial domain. The simplest (too
simple for many applications) is to just flip the lowest-order bit of
chosen pixels in a gray scale or color image. This will work well only if
the image will not be subject to any human or noisy modification.
A more robust watermark can be embedded in an image in the same way that a
watermark is added to paper. Such techniques may superimpose a watermark symbol
over an area of the picture and then add some fixed intensity value for the
watermark to the varied pixel values of the image. The resulting watermark may
be visible or invisible depending upon the value (large or small, respectively)
of the watermark intensity. One disadvantage of spatial domain watermarks is
that picture cropping (a common operation of image editors) can be used to
eliminate the watermark.

Spatial watermarking can also be applied using color separation. in this
way, the watermark appears in only one of the color bands. This renders the
watermark visibly subtle such that it is difficult to detect under regular
viewing. However, the watermark appears immediately when the colors are
separated for printing or xerography. This renders the document useless to the
printer unless the watermark can be removed from the color band. This approach
is used commercially for journalists to inspect digital pictures from a
photo-stockhouse before buying un-watermarked versions. Watermarking
can be applied in the frequency domain (and other transform domains) by first
applying a transform like the Fast Fourier Transform (FFT). In a similar manner to spatial domain
watermarking, the values of chosen frequencies can be altered
from the original. Since high frequencies will be lost by
compression or scaling, the watermark signal is applied to
lower frequencies, or better yet, applied adaptively to frequencies
that contain important information of the original picture (feature-based
schemes). Since watermarks applied to the frequency domain will be dispersed
over the entirety of the spatial image upon inverse transformation, this method
is not as susceptible to defeat by cropping as the spatial technique. However,
there is more of a tradeoff here between invisibility and decodability, since
the watermark is in effect applied indiscriminately across the spatial image.

Watermarking can be applied to text images as well. Three proposed methods
are: text line coding, word space
coding, and character encoding. For text line coding,
the text lines of a document page are shifted imperceptibly up
or down. For a 40-line text page, for instance, this yields
2**40 possible codewords. For word-shift coding, the spacing
between words in a line of justified text is altered (see Figure 3). For
character coding, a feature such as the endline at the top of a letter, "t"
is imperceptibly extended. An advantage of these methods over those applied to
picture images is that, by combining two or three of these to one document, two
documents with different watermarks cannot be spatially registered to extract
the watermark. Of course, the watermark can be defeated by retyping the text.

Figure 3.
An illustration of textual watermarking. Note that the character "f" is
shifted subtly to the left. this makes the watermark imperceptible under
normal reading conditions.

LIMITATIONS OF DIGITAL WATERMARKING

As of this writing, a counterfeiting scheme has been demonstrated for a
class of invertible, feature-based, frequency domain, invisible watermarking
algorithms. This counterfeiting scheme could be used to subvert ownership
claims because the recovery of the digital signature from a watermarked image
requires a comparison with an original. The counterfeiting scheme works by
first creating a counterfeit watermarked copy from the genuine watermarked copy
by effectively inverting the genuine watermark. This inversion creates a
counterfeit of the original image which satisfies two properties: (a) a
comparison of the decoded versions of both the original and counterfeit original
yields the owner's (authorized) signature, and (b) a comparison of decoded
versions of both the original and counterfeit original yield the forged
(inverted) signature. This, the technique of establishing legitimate ownership
recovering the signature watermark by comparing a watermarked image with the
original image breaks down. It can be shown that both the legitimate signature
and counterfeiter's signature inhere in both the watermarked and counterfeit
watermarked copies. Thus, while it may be demonstrated that at least one
recipient has a counterfeit watermarked copy, it can not be determined which it
is.

This research suggests that not all watermarking techniques will be useful
in resolving ownership disputes in courts of law. There will likely be
non-commercial applications, or those with limited vulnerability to theft, where
"good enough watermarking" will suffice. More sensitive applications
may require non-invertable or non-extracting watermarking techniques. These
issues are under consideration at this writing.

THE FUTURE OF WATERMARKING

Though publishers have been clamoring for some means to protect
their material on electronic networks, there has been no rush yet
to embrace any of the current schemes. This could be just due to
a period of inspection and appraisal, but our opinion is that
publishers and scientists have yet to fully understand the practical
specifications associated with the problem. Should the watermarks be visible or
invisible? What constitutes invisibility? How difficult should it be to remove
watermarks from images? How might one characterize "Good-Enough
Watermarking" for different commercial and non-commercial applications?
What constitutes a "reasonable" level of photo-editing? Or of
degradation? Can the original image be required for decoding? Is transferal of
the watermark from the electronic medium to the printed medium important? How
are the watermarks to be policed? Etc.

As scientists propose solutions and publishers experiment with them and
debate their merits, some methods of watermarking will emerge as useful and
widely used. When that happens, there will also be the emergence of external
agencies for monitoring electronic copyright infringement (much the same as
there are agencies for music and print copyright management). In the meantime,
the challenge is for the scientists to develop ever more invisible, decodable,
and permanent watermarking methods, and perhaps to meet even more specifications
as they are demanded.